UNITED NATIONS (CNN) --U.N. inspectors found no evidence before the U.S.-led invasion in March that Iraq had reconstituted its chemical, biological or nuclear weapons programs, chief U.N. inspector Hans Blix said Thursday.

The comments come as U.S. President George W. Bush, British Prime Minister Tony Blair and Spanish Prime Minister Jose Maria Aznar face mounting criticism from lawmakers in their countries over the weapons issue. (Critics blast Blair, Spain's Aznar pressed on WMDs)

"The commission has not at any time during the inspections in Iraq found evidence of the continuation or resumption of programs of weapons of mass destruction or significant quantities of proscribed items, whether from pre-1991 or later," Blix told the U.N. Security Council in what is expected to be his final report.

But he also said that the former Iraqi regime was unable to account for chemical or biological weapons it claimed to have destroyed and that weapons inspectors were unable to clear up discrepancies before leaving Baghdad in advance of the invasion.

"This does not necessarily mean that such items could not exist. They might. There remain a long list of items unaccounted for," Blix said. "But it is not justified to jump to the conclusion that something exists just because it was unaccounted for."

A U.S.-led invasion toppled Iraqi leader Saddam Hussein in April, but U.S. experts have yet to find the banned weapons the Bush administration said existed and posed a threat to the United States.

U.N. inspectors left Iraq the day before the invasion began in March, and the United States has expressed no interest in letting them return now that its troops control the country.

Blix said that Saddam's totalitarian rule had raised questions about the credibility of interviews of Iraqi scientists.

"I trust that in the new environment in Iraq in which there is full access and cooperation and in which knowledgeable witnesses should no longer be inhibited to reveal what they know," he said, "it should be possible to establish the truth we all want to know."

Before the war, President Bush and other administration officials said Iraq's suspected weapons programs and ties to al Qaeda posed a threat to the United States.

But nearly two months after the collapse of Saddam's government, all that has turned up are two trailers U.S. experts believe could have been used as mobile biological weapons laboratories. No such weapons were found in those facilities.

Calls for investigation

Sen. Jay Rockefeller, the ranking Democrat on the U.S. Senate Intelligence Committee, has called for an investigation into the Bush administration's arguments behind the war in Iraq and whether the evidence that bolstered those arguments was handled properly.

"The question is that we went into this war based upon primarily ties with al Qaeda on the part of Iraq and weapons of mass destruction posing an imminent threat to our country," Rockefeller, D-West Virginia, told CNN. "And so we need to know whether that intelligence was in fact accurate and whether it was done in the proper manner."

Administration officials have denied slanting intelligence to justify the war.

"I personally never asked anybody in the intelligence community to change a single thing that they presented, and I am not aware of any other official in this administration who did that," John Bolton, undersecretary of state for arms control, said Wednesday at a House International Relations Committee hearing.

U.S. Sen. Pat Roberts, chairman of the Senate Intelligence Committee, said Thursday that he doesn't think hearings are needed immediately.

"Let's do our homework first," said Roberts, R-Kansas.

Roberts and Senate Armed Forces Committee Chairman John Warner, R-Virginia, have said their panels first would review hundreds of pages of documents. Roberts said he is sure the United States will determine what happened to Iraq's weapons.

"Yes, there's a lot of pressure. There should be undue pressure," he said. "We need to find the weapons of mass destruction. They represent a grave threat."

But Rockefeller said the Bush administration's doctrine of pre-emptive war against countries it says pose a threat "puts an absolute premium on having superb intelligence."

"The intelligence has to be well-done. It has to be unbiased. It has to be untampered with," he said. "And it may very well be, but we need to know that."

Blair pressed

Blair, Bush's strongest ally in the Iraq war, has faced questions about whether he misled the British public about Saddam's weapons program -- particularly allegations that Iraq had chemical and biological weapons ready to use against advancing troops within 45 minutes.

The Financial Times reported Thursday that the source of that claim was a senior Iraqi military officer on active duty. Government officials told the newspaper they relied on the information and distributed it because the official was a senior figure in Saddam's regime, not a defector. (Full story)

Democratic presidential challengers also have begun to question the Bush administration's arguments for war.

U.S. Sen. Bob Graham of Florida said this week that the failure to find banned weapons in Iraq will raise "a serious question as to the credibility of the United States in the world and undercutting of confidence of the American people in the veracity of their government."

Former Vermont Gov. Howard Dean used the issue Thursday to criticize the Bush administration and rival Democrats who supported the war, such as former House Minority Leader Richard Gephardt of Missouri and Sens. John Kerry of Massachusetts and Joseph Lieberman of Connecticut.

"Mr. President, where are the weapons that you told us about?" Dean asked in an appearance before Democratic activists in New York.

A 1,200-member Pentagon survey team is being dispatched to the Persian Gulf to continue the hunt for Iraq's suspected weapons. Its leader, Maj. Gen. Keith Dayton, said last week that he thinks credible evidence exists but doesn't know why such weapons haven't been found.

"Things could have either been taken and buried, they could have been transported and they could have been destroyed," Dayton said. "It doesn't mean they weren't there when we thought they were there."

Dayton's team also will have responsibility for finding terrorists, war criminals and prisoners of war.